A major source of parasitic spectral distortion in spectroscopy systems arises from speckle. This effect is observable in the emission of lasers: a laser beam observed scattered from a surface, such as a piece of paper, will appear to have randomly distributed bright speckles or dots, which can change with time and wavelength. This speckle pattern is the result of phase interference patterns that form at the detector plane, such as the eye observing the laser spot, and is partly a function of the minimum spectral resolution of the system and the roughness of the scattering (reflecting) surface at which the laser emission is directed. The speckle phenomenon is caused by the simultaneous spatial and spectral coherence of the light source, where spatial coherence refers to the small spatial extent, or diameter, of the light source, and spectral coherence refers to the small spectral width of the light source. In spectroscopy systems, spectral coherence of the light source is determined by the spectral resolution of the system, whether the spectral selection is done before or after interaction with the sample, as in pre-dispersive or post-dispersive spectroscopy systems, respectively.
In the context of spectroscopy systems, the speckle affects the total power detected by a finite aperture, and/or finite numerical aperture, detector. Specifically, when speckle is present, the detected power is a function not only of the total intensity of the beam but how the speckle spots are distributed over the detector aperture and how those spots change with time and wavelength.
While speckle can even be observed in nominally incoherent (low coherence) sources, the biggest challenges arise when using coherent sources, such as lasers. The high coherence of the sources increases the contrast in the speckle pattern. This results in increased signal distortion in the detector's response. Since the speckle pattern changes apparently randomly with the changes in wavelength for a tunable laser in a tunable source spectroscopy system, the speckle has the effect of generating spectral distortion that erodes the signal to noise ratio (SNR) of the spectroscopy system. Whereas noise varies randomly with time and thus time averaging reduces noise and improves signal-to-noise ratio of the system, the speckle induced signal distortion does not vary randomly with time and thus time averaging does not reduce speckle signal distortion.